Recent developments in the fabrication of liquid crystal display devices have been directed toward materials which display images by exploiting the light scattering properties of liquid crystal entrapped in discrete quantities in a matrix. Such materials avoid the sealing problems encountered in conventional cell-type displays and make possible the fabrication of displays with a larger surface area than achievable with cell-type displays. Proposed types of materials include materials containing encapsulated liquid crystals, and materials with micropores into which liquid crystals are imbibed.
One prior art proposal for encapsulating liquid crystals is disclosed in French Patent No. 2,139,537 and involves forming an aqueous emulsion of nematic or cholesteric liquid crystal material with an immiscible binder such a polyvinyl alcohol. The mixture is emulsified in a high speed blender or the like to form droplets of the liquid crystal that are encapsulated by the binder. The encapulated droplets are then coated onto a clear plastic substrate having the usual conducting electrodes. A similar technique is described in U.S. Pat. No. 4,435,047.
A prior art proposal involving filling the open or connected micropores of a plastic sheet with a nematic or other type of liquid crystal is disclosed in U.S. Pat. No. 4,048,358.
Electrical manipulation of these materials between light scattering and light transmissive modes causes the materials, or imaging forming segments thereof, to appear opaque in one state and transparent in another state. Thermal manipulation of the materials by the application of sufficient heat to induce a transition from the liquid crystalline, light scattering state to the isotropic, light transmissive state causes the materials to switch from an opaque to a clear state.
Preferred liquid crystals for light scattering displays are of the nematic or smectic type and may have either positive or negative dielectric anisotropy. Nematic liquid crystals are, of the two types of liquid crystals, most readily responsive to applied fields because nematics display the least order, have the fewest constraints on molecular orientation, and generally have the lowest vicosity. Light scattering liquid crystals have an extraordinary index of refraction n.sub.e measured along their long axis which is greater than their ordinary index of refraction n.sub.o measured in a plane perpendicular to that axis. The long axis defines the optic axis of the liquid crystal. Light scattering liquid crystals having positive dielectric anisotropy respond to an applied electric field by aligning their optic axes parallel to the direction of the field; those having negative dielectric anisotropy respond by aligning their optic axes perpendicular to the field direction.
Light incident upon materials containing discrete areas of liquid crystals is either scattered or transmitted depending upon the relationship among the indices. For example, in devices employing nematic liquid crystals with positive dielectric anisotropy, the matrix is formed from a resin having an index of refraction n.sub.s substantially equal to the ordinary index of refraction n.sub.o of the liquid crystal. In the absence of an applied field, the liquid crystals entrapped in the generally spherical droplets have no preferred direction in which to align so that incident light encounters a mismatch between the index n.sub.s of the resin and the extraordinary index n.sub.e of the liquid crystal and is scattered. Application of a field causes alignment of the molecules with a resultant alignment of the extraordinary indices (optic) axes for each discrete quantity of liquid crystal. Alignment of the optic axes normal to a surface upon which light is incident causes the droplets to present a refractive index of n.sub.o for that light; since n.sub.o is essentially equal to n.sub.s, the incident light detects no mismatch between the indices and is transmitted so that the material appears clear. Such devices are disclosed in co-pending U.S. application Ser. No. 776,831.
Another operational principal of light scattering devices employing nematic liquid crystals having negative dielectric anisotropy is dynamic scattering. In these devices an applied voltage sets up a turbulence in the liquid crystals which scatters incident light. The visual effect of applying an external field to a dynamic scattering-type device is opposite to that described above. These devices display no scattering and are transparent in the absence of an applied field, while in the presence of a field, scatter and appear opaque. The scattering is due to the turbulence created in the liquid crystals by the applied field. Such a device is disclosed in French Patent No. 2,139,537.
Images may be displayed on light scattering and dynamic scattering liquid crystal devices by sandwiching a sheet or film of liquid crystal in a matrix between conductive electrodes and selectively energizing various segments or picture elements in known fashions to form alphanumeric character displays. As shown in U.S. Pat. No. 4,435,047, the picture elements of a nematic with positive dielectric anisotropy liquid crystal display to which a voltage is applied appear clear whereas segments without an applied voltage remain opaque; and in French Patent No. 2,139,537 the segments of a nematic with negative dielectric anisotropy liquid crystal device exhibit the opposite effect. A variable scattering device which exhibits images in a gray scale as in U.S. Pat. No. 4,411,495 is possible by varying the intensity of the applied segmental electric field to vary the difference in that segment between the effective indices of refraction of the liquid crystal and the matrix. The greater this difference, the more scattering and more opaque the segment.
The preparation of most of these displays involves the mechanical entrapping of discrete quantities of liquid crystals in a transparent matrix. Mechanical entrapment by encapsulation has the drawback of forming a relatively broad range of droplet diameters, so that sieving or sizing may be necessary to achieve uniform capsule sizes. Mechanical entrapment by imbibing into uniformly sized micropores overcomes the problem of diameter variation, but presents sealing difficulties. Furthermore, the operation of most of these types of liquid crystal displays depends upon a constant application of an external field, either electric or thermal, to maintain an image. While this mode of operation is desirable for displays of time and temperature, for instance, in which sundry alphanumeric characters are created and subsequently erased by the constant energizing and de-energizing of various picture elements, it would be advantageous in many instances to have a display technology characterized not only by greater ease of preparation but also by image display not dependent upon the constant presence of an applied field.
Co-pending U.S. patent application Ser. No. 776,831, the disclosure of which is incorporated by reference, describes a material comprising droplets of liquid crystal dispersed in a solid, cured thermosetting resin prepared by dissolving the liquid crystals in an uncured resin, such as epoxy, and curing the resin so that droplets of liquid crystal spontaneously form throughout the resin during curing.
As distinguished from prior art describing mechanical entrapment of light scattering liquid crystals U.S. patent application Serial No. 776,831 describes a material having the advantages of ease of preparation, control of droplet size and theoretically unlimited display size.